1. Field of the Invention
The present invention relates to an engine that actively varies a compress expansion ratio, which has a variable valve duration unit that improves an air charging efficiency, reduces a pumping loss, improves an expansion efficiency, and that varies a real compression ratio.
2. Description of Related Art
A typical internal combustion engine that is used for a vehicle includes a variable valve timing unit that varies a lift timing of an intake valve and an exhaust valve according to an engine driving condition to improve engine output and exhaust characteristic.
Meanwhile, researches for improving air charging efficiency, reducing a pumping loss, improving expansion efficiency, and varying a real compression ratio have been continuously undertaken.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.